As a flow rate sensor that measures the flow rate of a gas, a sensor that includes a flow rate detection unit for measuring the flow rate is known. Such a flow rate sensor is adapted to measure the flow rate of the gas by performing transmission of heat between the flow rate detection unit and the gas to be measured, which is the subject of measurement. Flow rates that are measured by such flow rate sensors are widely employed as important control parameters for devices of various types. While such heat type flow rate sensors are known to be capable of measuring the flow rates of gases at relatively high accuracy as compared to flow rate sensors of other types, in recent years, enhancement of the accuracy for measuring gas flow rate has become desirable. For example, with a vehicle to which an internal combustion engine is mounted, the demands for low fuel consumption and for exhaust gas purification are high. In order to satisfy such demands, there is a requirement for measurement at high accuracy of the amount of intake air, which is a principal parameter of an internal combustion engine.
A flow rate sensor that measures the amount of intake air conducted into an internal combustion engine includes an auxiliary passage that takes in a portion of the intake air amount, and a flow rate detection unit that is disposed in this auxiliary passage. The state of the gas to be measured as it flows in the auxiliary passage is measured by performing transmission of heat between the flow rate detection unit and the gas to be measured, and an electrical signal that specifies the amount of intake air being conducted into the internal combustion engine is outputted (for example, refer to PTL1).
An inlet that takes in some of the gas to be measured from a main passage through which it is flowing to the internal combustion engine, and an outlet that discharges to the main passage the gas that has been conducted from the inlet via the auxiliary passage to the flow rate detection unit, are provided in the casing of the flow rate sensor (refer to PTL1).